1. Field of the Invention
The invention relates to a coaxial cable which is widely used in an information communication field and particularly to a fine-diameter coaxial cable, with an outer diameter of an insulation of not more than 1 mm, adapted for the transmission of high frequency signals, and a coaxial multicore cable using the same.
2. Description of Related Art
For example, as shown in FIG. 1A, a conventional coaxial cable has a structure comprising: an inner conductor a, an insulation b, formed of polyethylene or the like, covering the circumference of the inner conductor a; an outer conductor c, formed of a braided metal or the like, provided on the circumference of the insulation b; and an insulating jacket d covering the circumference of the outer conductor c. In general, in the so-called xe2x80x9clow-attenuation coaxial cablexe2x80x9d used at a high frequency of 1 GHz to 10 GHz, as shown in this drawing, a plastic tape e with a metal layer is lengthwise attached to or wound around the circumference of the insulation b from the viewpoint of improving electric characteristics such as shield effect and attenuation level.
As shown in FIG. 1B, the construction of the plastic tape e with a metal layer is generally such that a metal foil g such as copper, aluminum, or silver is bonded to the surface of a plastic tape body f formed of a polyester, Teflon (registered trademark) or the like. When the metal foil g is formed of copper, the thickness is generally not less than 8 xcexcm, i.e., is larger than the thickness of the metal foil g formed of aluminum or silver which is 4 xcexcm.
In winding the conventional plastic tape e, with a metal layer, on the insulation b, when the insulation b is thick, no particular problem occurs. On the other hand, when the insulation b is very thin, for example, when the outer diameter is small and is not more than 1 mm, the winding work becomes very difficult and, in addition, after winding, a gap is formed between the plastic tape e and the insulation b, often leading to a deterioration in electric characteristics. Further, since the adhesion between the metal foil g and the tape body f is so small that, upon the application of external force, for example, as a result of flexure of the cable or friction against the outer conductor c, in the worst case, the metal foil g is separated from the tape body f. In particular, this phenomenon is significant when a highly flexible braided material is adopted as the outer conductor c.
To overcome this problem, when winding of the plastic tape on an insulation b having a small outer diameter of not more than 1 mm is contemplated, in order to form a relatively flexible plastic tape e with a metal layer, the formation of a metal layer on a tape body f by vapor deposition has also been proposed (for example, Japanese Patent Laid-Open No. 232611/1989 corresponding to U.S. Pat. No. 4,970,352). When this metal layer is formed by vapor deposition, however, the upper limit of the thickness of the metal layer is generally about 0.1 to 0.3 xcexcm in the case of copper and about 0.05 to 0.5 xcexcm in the case of aluminum. These thicknesses are unsatisfactory for providing desired electric characteristics.
Specifically, in order to attain satisfactory skin effect by a metal layer formed of copper or silver, a thickness of at least 2 xcexcm is required for a high frequency of 1 GHz, and a thickness of at least 1 xcexcm is required for a high frequency of 5 GHz. The vapor deposition method, however, cannot realize increased metal layer thickness without difficulty and thus disadvantageously cannot provide satisfactory electric characteristics.
Accordingly, the invention has been made with a view to solving the above problems of the prior art, and it is an object of the invention to provide a coaxial cable and a coaxial multicore cable which can effectively avoid the deterioration in electric characteristics attributable to the plastic tape with a metal layer.
According to the first feature of the invention, a coaxial cable comprising: an inner conductor; an insulation covering the inner conductor; a plastic tape, with a metal layer, provided on the insulation; an outer conductor provided on the plastic tape with a metal layer; and a jacket provided on the outer conductor, wherein the plastic tape with a metal layer comprises a tape body and, provided as the metal layer on the surface of the tape body in the following order, a first metal layer and a second metal layer formed of electroplating.
According to this construction, the thickness of the metal layer can be at least brought to a desired one, and, thus the deterioration in electric characteristics attributable to the plastic tape with a metal layer can be effectively avoided. Further, the tape per se can maintain the relatively flexible state. Therefore, even when the outer diameter of the insulation is as small as not more than 1 mm, the plastic tape with a metal layer can be easily and surely attached lengthwise to or wound around the circumference of the insulation. In addition, the adhesion between the metal layer and the tape can be improved. By virtue of this unfavorable phenomena such as separation of the metal layer can be prevented.
More specifically, when the first metal layer is formed by vapor deposition of a metal, the adhesion between the second metal layer formed of electroplating and the tape body can be improved. Further, when the thickness of the metal layer in the plastic tape with a metal layer is brought to more than 1 xcexcm and not more than 4 xcexcm, an increase in hardness of the tape can be surely avoided while enjoying satisfactory electric characteristics.
The adoption of a construction, wherein the plastic tape with a metal layer is provided so that the surface of the metal layer is in contact with the outer conductor, or a construction, wherein the plastic tape with a metal layer is constructed so that the metal layer is provided on both sides of the tape body can prevent unfavorable phenomena such as the separation of the metal layer from the tape body, even when the outer conductor is formed of a braided material, i.e., has concaves and convexes in a network form on its surface.
According to the second feature of the invention, a coaxial multicore cable comprises: a plurality of coaxial cables of any one of the above types, which have been twisted together; and, integrated with the coaxial cables, a sheath covering the circumference of the plurality of twisted coaxial cables.
This coaxial multicore cable possesses excellent electric characteristics and can be easily produced.
Coaxial cables, to which the invention is applied, preferably have an inner conductor size of 40 to 28 AWG (outer diameter: about 0.08 to 0.32 mm).